David E. Steitz
Headquarters, Washington, DC June 26, 1998
(Phone: 202/358-1730)
Mary Hardin
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-0344)
RELEASE: 98-113
NEW SATELLITE DATA SHOW RETREAT
OF EL NINO, PACIFIC OCEAN IN TRANSITION
New sea surface height measurements taken by the ocean-observing
TOPEX/Poseidon satellite show the equatorial Pacific in a state of flux
with the warm, high sea level El Nino-spawned waters in retreat and
areas of colder, low sea level waters on the increase.
"Sea level is a measure of the heat stored in the ocean. In the
last month or so, the tropical Pacific has been switching from warm to
cold. Lower sea level indicates less heat, hence a colder ocean," said
Dr. Lee-Lueng Fu, the project scientist for the U.S.-French
TOPEX/Poseidon mission at NASA's Jet Propulsion Laboratory (JPL),
Pasadena, CA. "It appears now the central equatorial Pacific Ocean will
stay colder than normal for some time to come because sea level is about
seven inches below normal, creating a deficit in the heat supply to the
surface waters. It is not clear yet, however, if this current cooling
trend will eventually evolve into a long-lasting La Nina situation."
An El Nino condition begins when steady westward blowing trade
winds weaken and even reverse direction. This change in the winds
allows a large mass of warm water that is normally located near
Australia to move eastward along the equator until it reaches the coast
of South America.
This displaced pool of unusually warm water affects evaporation,
where rain clouds form -- and, in turn, alters the typical atmospheric
jet stream patterns around the world. The change in the wind strength
and direction also impacts global weather patterns. The climatic event
has been given the name El Nino, a Spanish term for "the Christ child,"
because the warm current first appeared off the coast of South America
around Christmas.
The 1997-98 El Nino has been the strongest ever recorded. This
phenomenon was responsible for record rainfall in California, heavy
flooding in Peru, drought and wildfires in Indonesia, tornadoes in the
southeast United States and loss of life and property damage worldwide.
TOPEX/Poseidon's sea surface height measurements have provided
scientists with their first detailed view of how El Nino's warm pool
behaves because the satellite measures the changing sea surface height
with unprecedented precision.
A "La Nina" (Spanish for "little girl") is essentially the opposite
of an El Nino, where the trade winds are stronger than normal and the
cold water that normally exists along the coast of South America extends
to the central equatorial Pacific. A La Nina situation also changes
global weather patterns and is associated with less moisture in the air,
resulting in less rain along the coasts of North and South America.
TOPEX/Poseidon will be able to track a potentially developing La Nina
with the same accuracy.
"It may be too soon to say 'good-bye' El Nino and 'hello' La Nina,
because the effects of El Nino will remain in the climate system for a
long time," said Dr. Bill Patzert, a research oceanographer at JPL.
"However, if the Pacific is transitioning to a La Nina, we'd expect to
see clear, strong indication of it by late summer or early fall -- in
approximately August or September -- just like we did last year with El
Nino. The strongest impacts of a potential La Nina wouldn't be felt in
the U.S. until next winter." A La Nina does not automatically follow an
El Nino, Patzert added.
Developed by NASA and the Centre National d'Etudes Spatiales
(CNES), the TOPEX/Poseidon satellite uses an altimeter to bounce radar
signals off the ocean's surface to get precise measurements of the
distance between the satellite and the sea surface. These data are
combined with measurements from other instruments that pinpoint the
satellite's exact location in space. Every ten days, scientists produce
a complete map of global ocean topography, the barely perceptible hills
and valleys found on the sea surface.
Ocean temperatures affect ocean topography, which is why the
TOPEX/Poseidon radar altimeter is able to monitor the changing El Nino
and La Nina conditions. With detailed knowledge of ocean topography,
scientists can then calculate the speed and direction of worldwide ocean
currents.
The new satellite image from June is available at URL:
http://www.jpl.nasa.gov/elnino
JPL, a division of the California Institute of Technology, manages
the TOPEX/Poseidon mission for NASA's Earth Science Enterprise,
Washington, DC.
-end-